The prior art in the field of piezoelectric hydrophones is illustrated for example by patents FR-1,556,971; 2,122,675; 2,733,831 or 2,748,183 or by U.S. Pat. Nos. 3,970,878; 4,336,639 or 4,926,397; 5,541,894 or 5,815,466.
It is well-known to form a hydrophone by assembling elements sensitive to pressure variations, consisting each of a disk made of a piezoelectric ceramic, associated with a pair of electrodes arranged on either side. Each disk is stuck onto a flexible support such as a diaphragm, one face of which is exposed to the pressure variations to be measured.
The flexible support is for example a diaphragm supported by a rigid housing or the central part of a cup whose peripheral part is reinforced and rigid, resting against an identical cup, itself carrying one or more sensitive elements, the two cups delimiting a housing. The electrodes of the two sensitive elements are electrically interconnected so as to compensate for parasitic effects due to accelerations.
When the outside static pressure increases, the two plates bend until they rest against each other. Their spacing is so selected that their maximum deformation, when they are pressed against each other, remains within the elastic deformation limits. The pickup is thus protected against accidental overpressures. The housing thus formed can be coated with a layer of a material transparent to acoustic waves.
The sensitive elements can be externally fastened to the housing and provided with a protective coating (such as a varnished araldite layer) so as to maintain a sufficient electric insulation between the electrodes. The sensitivity of this type of hydrophone is good and does not vary much (less than 10% for a static pressure of 10 MPa for example) with the hydrostatic pressure.
According to another well-known layout, the sensitive elements are fastened to the inner faces of the cups and therefore inside the housing, which provides good protection against the outside medium. However, with this layout, the piezoelectric sensitive elements, which are sintered ceramic disks and therefore relatively fragile, may be damaged through crushing when the hydrostatic pressure increase presses them against each other by bending the plates. The sensitivity of hydrophones with sensitive elements inside the housing is lower and it greatly decreases when the hydrostatic pressure increases, a phenomenon attributed to their being fastened to a face that becomes convex.
It is also well-known to combine the two previous types in a hydrophone with two diaphragms delimiting a closed housing, each one carrying a pair of sensitive elements, one outside the housing, the other inside, the two pairs being electrically interconnected in parallel so as to compensate for certain drawbacks inherent in the two modes, but this solution is relatively expensive.
A common drawback of symmetrically connected hydrophones with a single pair of sensitive elements, both inside or outside the housing, is, as shown in FIG. 9, their relatively great capacitance variation with the hydrostatic pressure. It increases with the pressure when the sensitive elements are inside (curve C.sub.int), whereas it decreases when they are outside (curve C.sub.ext).